Downloading a ROM for a game you do own, however, is hypothetically defensible–at least legally speaking. Broadly speaking, downloading a ROM for a game you do not own is not legal–just like downloading a pirated movie is not legal. Is downloading ROMs legal? We talked to a lawyer about this, actually. You’ll have to figure out how to get ROMs yourself.
Basically no one does this, however, and instead downloads ROMs from a wide collection of websites that, for legal reasons, we will not be linking to.
In theory, you could fill a collection this way. For old cartridge-based consoles, special pieces of hardware hardware makes it possible to copy games over to your computer. And where do ROMs come from? If a game comes on a DVD, like the PlayStation 2 or the Nintendo Wii, you can actually rip games yourself using a standard DVD drive to create ISO files.